The present invention relates to pattern cutting of sheet material by means of a high velocity abrasive fluid jet, and more particularly, to a novel apparatus for supporting a glass sheet or blank during the cutting thereof.
The use of a high velocity fluid jet for cutting, drilling and abrading various materials is well-known in the art. The system employed generally comprises a source of high pressure fluid, such as a high pressure intensifier, for pressurizing the fluid in a chamber to a very high level (e.g., on the order of 50,000 PSI or more), a conduit to transport the fluid to the cutting location, and a nozzle with a very small orifice (e.g., 0.001-0.040 inch; 0.025-1.016 mm) through which the liquid is discharged in the form of a high velocity, small diameter cutting jet. The resulting jet is a highly collimated stream with very little dispersion that produces a relatively narrow kerf with a small volume of waste.
The process has been used successfully to provide a clean cut through a wide variety of soft materials such as plastic, wood and fibers, as well as some relatively hard materials as rock and metal alloys. It is also known to add abrasives, for example, silica sand, garnet, and the like, to the high velocity fluid jet to cut through some hard and brittle material, especially certain metals, ceramics, glass, etc., which cannot be cut using the conventional fluid jet.
In order to absorb the force or impact of the high velocity fluid jet it is necessary to provide a firm support for the material being cut. This is particularly important when making precision cuts in sheet-like articles or workpieces. Even the slightest movement of the sheet can cause a cut to deviate from the intended path, resulting in a defective cut. In the majority of cutting and drilling operations, the fluid jet penetrates completely through the thickness of the workpiece and into the support surface therebelow. This will, of course, have an erosive effect on the support surface. Repeated penetrations of the jet stream, randomly or especially in a recurring pattern, will wear away portions of the surface until eventually it no longer provides the firm support needed and has to be replaced. For the above reasons, such sheets are generally supported on an expendable or "sacrificial" plate, mounted on a rigid supporting frame, that can be replaced when the need arises. Support plates of plywood, Plexiglass.RTM., and the like, have served this need adequately in the past, especially when using a conventional water jet system. However, with the use of abrasives entrained in the fluid jet to increase cutting capabilities, the useful life of the sacrificial plates is appreciably shortened, requiring more frequent replacement. Although the plates are relatively inexpensive in themselves, the routine replacement, and consequent downtime and labor involved, add considerably to the cost of the operation.
Also, it has been found extremely critical when cutting certain fragile sheet materials, e.g., glass and some plastics, that the sheet be uniformly supported adjacent either side of the line of cut. This is to prevent the sagging of one side below the other during the cutting operation, creating undesirable stresses at the point of penetration of the liquid jet which can result in so-called vents or cracks along the finished edge. Even the smallest vents barely visible to the naked eye may be unacceptable, especially when the material is glass which is to undergo subsequent heat treatment for use in the automotive or architectural markets. Such heat treatment may include bending and/or annealing or tempering, and as is well known, when a vented glass sheet is exposed to such treatment, there exists a strong possibility the vent will spread and destroy the sheet. The loss of sheets, especially at this late stage of production, adds considerably to the overall costs and thus should be controlled as much as possible. Should a vented sheet survive the heat treatment and enter the market place there is the ever present danger that it may fracture on impact or even due to exposure to radiant heat from the sun. Under such conditions, tempered lights have been known to spontaneously fracture, or disintegrate with force, due to the release of residual stresses therein and scatter particles in all directions. Thus, the importance of providing vent-free glass sheets can readily be understood.
The aforementioned sacrificial plate employed heretofore has generally been mounted on a plurality of accurately aligned and closely spaced metal cross beams to provide a firm, level support surface for the sheet to be cut. While this structure initially provides adequate support, continued use and prolonged exposure to the fluid may produce temperature differentials in the support plate causing it to warp and develop an undulating or uneven support surface. Prolonged exposure to the fluid alone may have the same effect on some plates, such as the plywood frequently used. As previously mentioned, an uneven support surface has an adverse effect on the quality of the cut, particularly when the material is a sheet of glass.
Another area of concern exists when utilizing an abrasive fluid jet for cutting transparent sheets of glass and plastic that must meet stringent optical requirements. This can include glazing closures for buildings, aircraft, automobiles and the like, wherein the viewing area of the closure or window must be free of optical defects that would tend to interfere with clear viewing therethrough. As previously mentioned, the sheets are supported on the top surface of a sacrificial plate during the cutting operation and the cutting medium after penetrating through the sheet, will cut into the surface of the support plate. As the abrasive-containing fluid strikes the support plate, there will be a certain amount of splash-back or rebound of the abrasive material against the bottom surface of the glass, creating an undesirable gray or frosted area adjacent the line of cut. This frosted area is especially pronounced when the abrasive jet penetrates completely through the support plate and ricochets off one of the metal reinforcing beams immediately therebelow. Depending on the severity of the frosting, the glazing closure may not meet specified optical requirements and thus may be considered unacceptable.